1. Field of the Invention
The present invention relates to an internal combustion engine system which uses a blended fuel of gasoline and ethanol, and can switch back and forth between spark ignition combustion and homogeneous charge compression ignition combustion.
2. Description of the Related Art
In recent years, in order to improve fuel efficiency and reduce the emission of an internal combustion engine, a compression ignition internal combustion engine represented by a homogeneous charge compression ignition internal combustion engine has been studied. However, the above described homogeneous charge compression ignition internal combustion engine has a problem that the engine has a narrow stably-operatable region, because the engine easily causes knocking when a high load is required, and easily causes misfire when a low load is required.
In order to solve the above described problem, a homogeneous charge compression ignition internal combustion engine has been conventionally known which is an internal combustion engine using gasoline, and can switch back and forth between spark ignition combustion and homogeneous charge compression ignition combustion (cf. Japanese Patent Laid-Open No. 2002-130006 and Japanese Patent Laid-Open No. 2001-152919, for instance).
The above described homogeneous charge compression ignition internal combustion engine usually conducts the above described homogeneous charge compression ignition combustion, and conducts the above described spark ignition combustion only when a requested load is higher than usual or lower than usual. The above described spark ignition combustion has an advantage of being capable of forcefully igniting fuel with a spark at the optimal timing, but has a disadvantage of greatly decreasing thermal efficiency when using a single fuel, because of needing to delay ignition timing to later timing than the optimal timing for avoiding knocking when a high load is required.
On the other hand, it has been known to use a blended fuel of gasoline and ethanol as a fuel for an internal combustion engine. It has been known that the above described blended fuel can provide a so-called carbon neutral effect because the blended fuel uses ethanol obtained by fermenting and distilling plants, for instance, farm products such as sugarcane and corn. The above described carbon neutral effect means that the combustion of ethanol obtained from plants does not theoretically increase the amount of emitted carbon dioxide after all, because the plant of a raw material in itself has already absorbed carbon dioxide, and even when the ethanol obtained from the plant is combusted, it emits carbon dioxide in an equal amount to what the plant has already absorbed.
Accordingly, a blended fuel of gasoline and ethanol can reduce an amount of emitted carbon dioxide and contribute to the prevention of global warming, by being used for automotive fuel.
However, the above described blended fuel has a problem that the blended fuel cannot sufficiently develop the above described carbon neutral effect, because even though the ethanol uses plants, the ethanol also makes fossil fuel consumed in the production process and the distribution process. The above described blended fuel has also a problem of needing a higher cost than general gasoline fuel and making the movable distance of the automobile shorter than that of an automobile using the general gasoline fuel, when both automobiles have the same capacity of the fuel tank, because the ethanol has a heating value per unit volume in an amount of only a little over 60% of the general gasoline fuel.
As for a method of using the above described blended fuel, a technology has been already known which includes adding water to the blended fuel to separate the blended fuel into gasoline and ethanol, and using gasoline and ethanol each independently (cf. Japanese Patent Laid-Open No. 58-96155). However, the technology is a method of separating ethanol from the above described blended fuel, further reforming the separated ethanol into gas consisting of hydrogen and carbon monoxide, and using the reformed gas, and accordingly does not sufficiently use the blended fuel efficiently.
For this reason, the internal combustion engine is demanded which can efficiently use a blended fuel of gasoline and ethanol.